1. Technical Field
This invention generally relates to semiconductor devices, and more specifically relates to pull-up and pull-down circuits.
2. Background Art
Pull-up and pull-down circuits are used to pull-up and pull-down nodes to a DC level that are otherwise generally left unconnected when mounted to the final package. For example, pull-up circuits are commonly formed on semiconductor devices and connected to input nodes to allow the node to be pulled high without requiring an external circuit to pull the node up. The pull-up circuit can also be used to pull up a node that is externally connected but not driven. One use of a pull-up/pull-down circuit is in testing the semiconductor device. For example, a pull-up circuit can be used to pull a node high to test for excess leakage current. When testing is complete, these pull-up devices can be deactivated to allow the node to float for normal device operation.
Traditionally, P-channel transistors with the gate tied to ground are used in pull-up circuits while N-channel transistors with the gate tied to power supply are used in pull-down circuits.
The traditional P-channel transistor pull-up circuits with their gate tied to ground draw a current when being used to pull up a node that is being held externally to a low level. When the pull-up transistor circuit is used in a testing environment, the current can mask leakage current due to defects in the actual device and thus interfere with accurate testing.
Likewise, N-channel transistor pull-down circuits with their gate tied to power supply drain a similar current when used to pull down a node that is held externally to a high level. Again, this DC current path can interfere with testing measurements for leakage current on the device.
Because no acceptable solution has existed in the past, this problem is usually avoided by not measuring quiescent ground currents. However, it remains desirable to have the ability to measure quiescent current especially in diagnosing and debugging new designs. Thus, there remains a need for pull-up and pull-down circuits that bias a node without drawing DC current.